Thermal decomposition of high-nitrogen energeticcompounds—dihydrazido-S-tetrazine salts

Jimmie C. Oxley*, James L. Smith, Heng ChenChemistry Department, University of Rhode Island, Kingston, RI 02881, USA

Abstract

The thermal stabilities of 3,6-dihydrazido-1,2,4,5-tetrazine (Hz2Tz) and its salts with diperchlorate [Hz2Tz(HClO4)2],

dinitrate [Hz2Tz(HNO3)2], bisdintramidate [Hz2Tz(HDN)2], and bisdinitroimidazolate [Hz2TzBim] have been examined and

compared to other 3,6-disubstituted tetrazines. The neutral tetrazines exhibited two principal modes of decomposition:

elimination of N2 from the tetrazine ring followed by cleavage of the remaining N–N bond, and loss of the substituent group,

in some cases assisted by proton transfer. The salts Hz2TzX2 undergo reversible equilibrium with the parent Hz2Tz and HX,

thus, in several cases the decomposition rate of the parent tetrazine and the salt are essentially identical. # 2002 Elsevier

Science B.V. All rights reserved.

Keywords: 3,6-Dihydrazido-1,2,4,5-tetrazine (Hz2Tz); Tetrazine; Thermal stability; Thermal decomposition

1. Introduction

Traditional explosives, nitrate esters, nitroarenes,

nitramines, must contain sufficient NO2 groups to self-

oxidize the carbon and hydrogen atoms of the mole-

cule. In designing new energetic materials, one area of

emphasis has been on increasing the nitrogen content

at the expense of carbon/hydrogen content. This has

two advantages: lowering C/H content lowers the

oxygen requirement, substituting N–N for C–N usu-

ally increases the heat of formation. Central to these

efforts have been the study of nitrogen-containing

rings. In some cases the effort was successful, as in

the case of 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one

(NTO) [1–6]. Other compounds, such as 2,4,6-trinitro-

1,3,5-triazine have been proposed, but not success-

fully synthesized [7,8]. Tetrazine, a high-nitrogen ring

compound, is one of the frameworks on which new

energetic materials are based. The spectroscopic and

photodissociation properties of known tetrazines have

long been an area of interest to the theoretician.

Synthesis efforts at Los Alamos National Laboratory

(LANL) have resulted in a series of new 3,6-substi-

tuted-1,2,4,5-tetrazines, both neutral species and salts

[9–14]. We have examined and reported the thermal

stability of the neutral tetrazines—3,6-substituted-

1,2,4,5-tetrazines, where the substituents were Cl,

NH2, NH3NH2 and 3,5-dimethylpyrazole [15]. Now,

we report thermal studies on one of the salts of these

tetrazines—3,6-dihydrazino-1,2,4,5-tetrazine.

2. Experimental section

3,6-Dihydrazido-1,2,4,5-tetrazine (Hz2Tz) and its

diperchlorate [Hz2Tz(HClO4)2], dinitrate [Hz2Tz-

(HNO3)2], bisdintramidate [Hz2Tz�2HN(NO2)2], and

bisdinitroimidazolate [Hz2TzBim] salts were synthe-

sized and provided by Dr. Michael Hiskey of LANL

Thermochimica Acta 384 (2002) 91–99

* Corresponding author.

E-mail address: joxley@chm.uri.edu (J.C. Oxley).

0040-6031/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 4 0 - 6 0 3 1 ( 0 1 ) 0 0 7 8 0 - 8

[15]. Properties of these salts (Fig. 1) are listed in

Table 1. Thermal stability was initially evaluated

using differential scanning calorimetry (DSC). Sam-

ples (0.1–0.4 mg) were sealed in capillary tubes

(1.5 mm o:d:� 10 mm), which were held in an alu-

minum cradle [16] under nitrogen flow inside the head

of a TA Instruments 2910 (calibrated against indium).

Thermograms used for the comparison were obtained

over the temperature range 40–500 8C at a scan rate of

20 8C/min. In some cases, the American Society for

Testing and Materials (ASTM), differential heating

rate method was used to calculate Arrhenius para-

meters [17]. Thermograms were collected at heating

rates (b) of 1, 2.5, 5, 10, and 20 8C/min, and activation

Fig. 1. The tetrazine salts.

92 J.C. Oxley et al. / Thermochimica Acta 384 (2002) 91–99

energy (Ea) was calculated from the slope of the plot of

log10b versus 1/T (T is the exothermic peak tempera-

ture (exo. T) in K), as follows:

Ea ¼ �2:19Rdðlog10bÞdð1=TÞ

� �

where R is the gas constant.

A refinement of the activation energy and an esti-

mation of the Arrhenius pre-exponential factor (A)

were calculated according to the ASTM protocol,

where a table of D factors is given [17].

A ¼ bEaeEa=RT

RT2; refined

Ea ¼ �2:303R

D

� �dðlog10bÞdð1=TÞ

� �

Isothermal thermolyses were performed on samples

(0.1–0.6 mg) in tubes 4 mm o:d:� 50 mm to measure

kinetics or in narrower tubes (1.2–1.5 mm o:d:�50 mm) to determine decomposition gases. The con-

stant temperature bath consisted of an aluminum block

with four chambers containing molten Wood’s metal,

a hot plate acted as the main heating source, while the

temperature was maintained within 1 8C with a 250 W

immersion heater, an Omega (CA/500/J1) temperature

controlling sensor and Omega 651 resistance thermo-

meter. Rate constants were calculated by determining

the fraction of sample remaining at various times

during the isothermal heating. After the sample was

heated for a specified time interval, the tube was

broken, and the sample was transferred to a 10 ml

volumetric flask using distilled, dionized water. The

remaining quantity of dihydrazido-tetrazine (Hz2Tz)

was assessed by a Hewlett Packard (HP) 1084B high

performance liquid chromatograph (HPLC). The LC

was equipped with a 20 ml injection sample loop, a

Waters 486 Tunable Absorbance Detector (set to

250 nm), an Alltech Econosphere C18 5U column

(4:6 mm � 250 mm), and a HP 3396 series III inte-

grator. The mobile phase consisted of water and

methanol or acetonitrile, details are shown in Table 2.

To identify the decomposition gases, the sample

was sealed under vacuum or air and heated to com-

plete decomposition. The decomposition gases were

analyzed by gas chromatography/mass spectrometry

(GC/MS) [18]. A HP Model 5890 Series II GC,

equipped with Model 5971 electron impact quadruple

mass spectrometer was run in scan mode (mass range

12–200) with a threshold of 150 and a sampling of 2

(3.5 scans per second). Helium was used as the

carrier gas, and the GC column was a PoraPLOT Q

Table 1

Properties of dihydrazido-tetrazine saltsa

Sample Color Solubility

Unheated color DSC residue Isothermal

residue

Water Acetonitrile Methanol Acetone Density

(g/cm3)

H50

Hz2Tz Dark red powder Yellow residue Dark yellow or

black residue

Fair Insoluble Insoluble Insoluble 1.69 65

Hz2Tz(HClO4)2 Yellow powder None Black residue Good Good Good Good 1.96 14

Hz2Tz(HNO3)2 Orange powder None Black residue Good Insoluble Good Good 1.80 17

Hz2Tz(HDN)2 Yellowish green

powder

None Black residue Good Insoluble Good Good 1.82 11

Hz2TzBim Orange powder Dark gray residue Black residue Insoluble Insoluble Insoluble Insoluble 1.83 60

a Densitry and H50 from LANL data [15].

Table 2

HPLC solvents and flows for 3,6-dihydrazino-tetrazine salts

Sample Hz2Tz(HClO4)2 Hz2Tz(HClO4)2 Hz2Tz(HDN)2 Hz2TzHz2Tz(HNO3)2

Solvents 10% CH3OH in water 50% CH3OH in water 10% CH3CN in water 10% CH3CN in water

Flow rate (ml/min) 0.4 1.0 0.4 0.6

Retention time (min) 9.6 3.3 8.3 5.8

J.C. Oxley et al. / Thermochimica Acta 384 (2002) 91–99 93

(0:25 mm � 25 mm) from Chrompack. The GC injec-

tor temperature was 100 8C, and the detector/transfer

line temperature was 180 8C. Initially, the oven tem-

perature was held for 5 min at �80 8C, it was then

increased at a rate of 15 8C/min up to 190 8C. Decom-

position gases were identified by comparing their GC

retention times and mass spectra to authentic samples.

When authentic samples were not available, sample

spectra were compared with the NIST MS library for

tentative assignment.

Total amount of decomposition gases was initially

assessed by heating the tetrazine salts to complete

decomposition, inside a sealed capillary tube and

breaking the tube into a mercury manometer. To quan-

tify each of the decomposition gases, a HP 5890 series

II GC with a thermal conductivity detector (GC/TCD)

and Hayesep DB 100/120 (300 � 1=800) column (All-

tech) was used. A standard gas mixture—40% N2, 5%

CO, 30% CO2, and 25% N2O (Scott Specialty Gases)—

and gas sample loops of 10, 50, 100, 250 and 500 ml

volume were used for calibration. The injector tem-

perature was set at 120 8C, the detector temperature at

200 8C, flow of helium carrier gas was at 20 ml/min.

The oven was held for 6 min at 35 8C and then ramped

(40 8C/min) to 180 8C, where it was held 28 min.

Injection of the gaseous decomposition products

was accomplished by placing the sealed tubes in a

Nalgene (870 PFA, i.d. 1/800), sample loop in line with

the carrier gas and injector of the GCTCD or GC/MS.

The sample loop was purged with helium, the oven

was cooled to �80 8C, the sample tube was broken by

bending the flexible loop, and the loop was switched in

line. Although the Nalgene 870 PFA loop was perme-

able to air, because it was continuously under helium

gas pressure, air contamination was constant and

relatively low. To check the baseline, an empty capil-

lary tube, sealed according to the procedures for

samples, was broken in the GC loop [18].

3. Results

3.1. Kinetics

DSC was used to rate the relative thermal stabilities

of Hz2Tz and its nitrate [Hz2Tz(NO3)2], dintramidate

[Hz2TzDN2], perchlorate [Hz2Tz(ClO4)2], and imida-

zole [Hz2TzBim], salts. Three of the five exhibited

their major exotherm between 152 and 164 8C [Hz2Tz,

Hz2Tz(NO3)2, and Hz2TzDN2], the perchlorate salt

exhibited an exotherm about 308 higher (190 8C), and

the imidazole, even higher (220 8C) (Table 3). All the

salts released more heat than the parent neutral spe-

cies. Isothermal heating followed by LC analysis of

fraction of remaining tetrazine was used to determine

decomposition rate constants. The decompositions

were not first-order very far into their decompositions,

Hz2Tz and Hz2Tz(ClO4)2, were first-order to 40%

conversion, Hz2Tz(NO3)2 and Hz2TzDN2, only to

about 10%. The rate constants were calculated from

the linear portion of the curve, they are used for

comparison purposes only. The extent of the decom-

position curve used to calculate the rate constant is

shown as percent fraction reacted ‘‘F.R.%’’ in Table 3.

In line with the DSC results, Hz2Tz, Hz2Tz(NO3)2,

and Hz2Tz[N(NO2)2]2 exhibit similar rate constants

(0.002–0.003 s�1 at 140 8C), while the perchlorate salt

decomposed significantly slower. The decomposition

kinetics of Hz2TzBim were not determined by LC,

because it was insoluble in common LC solvents

(acetone, acetonitrile, methanol, water), although it

was soluble in DMF and DMSO. The decomposition

kinetics of Hz2TzBim were determined by the DSC

ASTM method [17]. Using Arrhenius parameters, a

rate constant of 3 � 10�6 s�1 could be calculated for

Hz2TzBim at 140 8C. The decomposition rate constant

was also estimated using gas manometry to determine

the total amount of gas produced with time relative

to the total amount of gas produced upon 100%

decomposition. That value was in good agreement

(2 � 10�6 s�1) with the one calculated from the

DSC data.

To examine the possible involvement of hydrogen

transfer during the decomposition process, some of the

tetrazine salts were decomposed in solution. Solutions

of Hz2Tz(NO3)2 and Hz2TzDN2 (�0.02 M) were pre-

pared in methanol and in water. The decomposition of

Hz2TzDN2 was first-order out to 60% conversion in

methanol and water, while it appeared autocatalytic in

the melt. This observation is not unusual. Autocata-

lytic behavior in the neat melt becomes first-order in

the presence of a solvent which removes the catalytic

decomposition product. Hz2TzDN2 was thermolyzed

in both proteo and deutero water. An external solvent

deuterium kinetic isotope effect (DKIE) of 1.8 was

observed.

94 J.C. Oxley et al. / Thermochimica Acta 384 (2002) 91–99

3.2. Products

Three of the tetrazine salts, [Hz2Tz(NO3)2],

[Hz2TzDN2], and [Hz2Tz(ClO4)2], left no residue

when heated in the sealed DSC tubes. The other

two, containing significantly less oxygen in their

structure, left a black residue. Isothermal heating of

the salts, even when taken to complete decomposition,

left a blackened residue in every case. Presumably, a

residue remained because the high temperatures used

in the DSC scans were never approached in any of the

isothermal tests. No remainder of the original tetrazine

ring was detected. However, each salt formed gaseous

decomposition products, and these were identified by

GC/MS and quantified by GC/TCD (Table 4). As in

the case of the neutral tetrazines [15], the principle

decomposition gas was N2. However, the neutral

tetrazines formed 0.5–1.5 mol gas per molecule, three

of the salts formed 6–13 mol of gas. We attribute the

extra gas to the anion itself, nitrate and dinitramidate

[8] make nitrogen and nitrous oxide, while Bim makes

CO. The anion does not oxidize more of the tetrazine

residue, otherwise the perchlorate salt would have

produced large quantities of gas, too (Table 4).

4. Discussion

Table 5 compares some of the observations we made

during the thermolysis of the tetrazine salts to those we

Table 3

DSC peaks and isothermal kinetics for tetrazine salts

Sample

Hz2Tz0 Hz2Tz(HNO3)2 in

CH3OH

Hz2Tz(HDN)2 Hz2Tz(HClO4)2 Hz2TzBim

CH3OH H2O D2O

DSC exo max 164 8C 1409 J/g 161 8C 5012 J/g 152 8C 5773 J/g 191 8C 279 J/g 222 8C 3998 J/g

208/min 299 8C 365 J/g 288 8C 1487 J/g

Temperature (8C) 434, 457 1500 ASTM Gas

170

k (s�1) 2.80E�04 1.4E�04 4.0E�05

F.R. (%) 37

160

k (s�1) 9.60E�04

F.R. (%) 43

140

k (s�1) 2.30E�03 2.80E�03 1.50E�04 1.80E�03 1.60E�03 1.40E�04 4.80E�05 4.8E�06 3.9E�06

F.R. (%) 63 90 95 88 34 37 59

130

k (s�1) 2.70E�04 6.00E�04 6.00E�05 3.3E�05

F.R. (%) 90 41 45 41

120

k (s�1) 3.30E�04 3.80E�04 2.00E�05 1.00E�05

F.R. (%) 65 92 95 75

110

k (s�1) 2.30E�05 5.20E�05

F.R. (%) 94 45

100

k (s�1) 5.00E�05 6.20E�05 2.10E�06 2.70E�06

F.R. (%) 64 90 96 78

80 2.50E�06

k (s�1) 4.10E�06

F.R. (%) 81 91

Ea (kJ/ml) isothermal 126 139 205 151 135 92.5

Ea (kcal/mol) 30 34 49 36 33 22

A (s�1) 2.20Eþ13 1.10Eþ15 1.20Eþ23 1.90Eþ16 2.00Eþ13 2.30Eþ07

R2 0.998 0.99 0.987 0.998 0.998 0.998

Ea (kJ/ml) ASTM 155 172

Ea (kcal/mol) 37.0 41

A (s�1) 2.51Eþ17 2.33Eþ16

R2 0.995 0.996

J.C. Oxley et al. / Thermochimica Acta 384 (2002) 91–99 95

made for the neutral tetrazines [15]. Four of the com-

pounds [Hz2Tz, Hz2Tz�2HNO3, Hz2Tz�2HDN, and

HzTzP (where P is 3,5-dimethylpyrazole)] exhibited

DSC exotherms around 160 8C, and their rate constants

at 140 8C were very similar ((2.5–5:2Þ � 10�3 s�1). For

the neutral tetrazines, we found that one mode of

thermal decomposition was the same observed under

electron impact: elimination of two nitrogen atoms as

N2 and cleavage of the remaining N–N bond (Fig. 2).

However, we observed that two of 3,6-substituted

neutral tetrazines, both of which had hydrazido sub-

stitutents in the 3/6-positions were far less stable than

the other tetrazines. This observation led us to conclude

that the main decomposition pathway involved disso-

ciation of the substituent group, in some cases assisted

by proton transfer. In the study of neutral tetrazines, we

found an external deuterium kinetic isotope effect

(DKIE) in cases where one substituent was amino.

We attributed this behavior to protonation enhancing

the rate of amino loss as NH3. Neither the diamino nor

Table 4

Moles of decomposition gas per mole tetrazine salt at complete decomposition

Hz2Tz Hz2Tz�2HNO3 Hz2Tz�2HDN Hz2Tz�2HClO4 Hz2Tz�Bim

Sample amount (mg) 0.78 0.70 0.55 0.60 0.17 0.19 1.42 1.00 0.74 0.65

Heating temperature (8C) 240 240 240 240 145 145 150 150 240 240

Heating time (min) 60 45 1560 210 210 50 1440 1200 30 50

Moles gas/mole Tz Salt

N2 0.64 0.65 4.27 4.79 6.46 7.27 1.13 1.03 6.93 6.88

CO2 0.14 0.17 0.98 0.85 1.30 1.40 0.47 0.67 0.50 0.64

N2O 0.04 0.38 0.32

CO 0.71 1.24 1.20 1.35 5.94 5.87

NO

Total gas by GC 0.79 0.83 6.0 6.9 9.5 10.4 1.6 1.7 13.4 13.3

Total gas by manometer 0.73 0.76 5.7 7.0 8.2 8.2 1.6 1.5 13.9 12.7

GC/MS Ret. time

(min)

Area of chromatogram (%)

N2 3.4 240 8C 27.4 240 8C 16.3 145 8C 20.8 150 8C 15.7 240 8C 28.3

O2 3.9 40 min – 210 min 1.3 50 min 4.7 1440 min 2.9 30 min –

CO 4.8 Trace 1.7 1.5 11.8

CO2 14.1 72.6 63.1 66.9 81.3 51.3

N2O 14.8 Some 15.6 3.9 1.8

H2O 19.5 Some 1.9 2.2 Some Some

HCN 21 – – – 6.9

CH3CN 25.5 Some Trace – Trace

Fig. 2. Decomposition scheme of 1,2,4,5-tetrazines.

96 J.C. Oxley et al. / Thermochimica Acta 384 (2002) 91–99

Table 5

Comparison of neutral tetrazines & salts

Salts of NH2NH-Tz-NHNH2 Group 1 Group 2 Group 3

2HNO3 2HN(NO2)2 2HClO4 Bim NH2NH-Tz-X P2TzH2 P2Tz PTzNH2 ClTzNH2 Cl2Tz (NH2)2Tz

X ¼ NHNH2 X ¼ P

DSC exo (8C) 161 152 191, 288 222 164 164 273 297 297 238 341 347

434, 457 Estimate 299 305

Rate 140 C � 10�3 2.8 1.8 4.8E�02 2.0E�06 2.5 5.2 1.2E�01 6.3E�02

DKIE Water 130 8C 1.8 Acetone 240 8C 1.4 1.2 1 2 1.5 – –

Total gas/Tz 6.4 8.2 1.6 13 0.8 1.2 0.6 0.7 0.7 0.9 1.5 1.2

N2/Tz 4.5 6.9 1.08 6.9 0.7 0.6 0.3 0.6 0.6 0.3 0.4 0.6

CO2/Tz 0.9 1.4 0.6 0.6 0

N2O/Tz 0.04 0.4 CO/Tz 5.9 0

Other products observed in GC/MS

CH3CN Trace Trace Some Small Small Small Small

HCN – 7% – Trace Trace Trace

NH3/water 2% Some Trace

J.C.

Oxley

eta

l./Th

ermo

chim

icaA

cta3

84

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99

97

the dihydrazido were tested, but undoubtedly both

would also have exhibited an external DKIE.

In this study of the salts of the dihydrazido-tetra-

zine, we found two (Hz2Tz�2HNO3, Hz2Tz�2HDN)

of the four salts had thermal stabilities similar to

the parent tetrazine, but two were substantially more

stable. If the decomposition pathway were totally

dependent upon loss of the substituent in the 3/6-

position, all should decompose at about the same rate.

As with the neutral tetrazines, we observed an external

DKIE when the bisdinitramidate salt was thermolyzed

in D2O and H2O. This could be due to protonation of

the hydrazido substituent as postulated for the neutral

tetrazines, but it could also be due to protonation of the

anion resulting in the reformation of the neutral

dihydrazido-tetrazine. Since loss of a 3/6 substituent

does not explain all the experimental results, the effect

of the anion must be considered.

We have previously reported a thermal stability study

conducted on a series of salts [NO3�, N(NO2)2

�, Cl�,

NTO�] of the nitrogen-heterocycle 3,3-dinitroazetidine

(DNAZ) [19]. In that study, we found their relative

thermal stability in water roughly followed the acid-

ity of the parent acid. The results suggested a mechan-

ism that involved proton transfer from the dinitroaze-

tidium cation to form DNAZ. DNAZ, regardless of the

salt of origin, then decomposed at approximately the

same rate. Table 6 shows some of the kinetics data from

the DNAZ study along with similar data from the

dihydrazido-tetrazine salts examined herein. The simi-

larities are obvious. The salts of nitrate and dinitrami-

date, which have similar acidities, decomposed at

about the same rate regardless of whether the cation

was DNAZþ or Hz2Tz2þ. In fact, one mode by which

the dinitramidate anion decomposes formed nitrous

oxide and nitrate [20]. Perhaps in studying Hz2Tz�2HDN, we are really examining the decomposition

of Hz2Tz(HNO3)2. The overall mechanism we envision

is shown below. Step 1 is an equilibrium for strong

acids, such as HClO4 and HCl, leans strongly to the left,

little of the neutral organic species is available. Once

formed the organic species DNAZþ or Hz2Tzþ rapidly

decomposes. In the case of the tetrazine, loss of a 3 or 6

substituent may be the mode of decomposition (step 2).

However, another possibility for 1,2,4,5-tetrazine

decomposition is shown in Fig. 2. In this case, we think

loss of the hydrazido ligand more likely though a few

cyanide species were observed in the decomposition

products.

½HL � Tz � LH�2þ þ 2X� , L � Tz � L þ 2HX

(1)

L�Tz�L !�L or HLN2;þL�CN; decomposition products

(2)

5. Conclusion

The salts Hz2TzX2 undergo a reversible equilibrium

with the parent Hz2Tz and HX, thus, in several cases

the decomposition rate of the parent and the salt are

essentially identical. The parent tetrazine compound,

once formed, can decompose by loss of N2 or by

the loss of the exocyclic substituent. The latter reac-

tion is aided by proton transfer, an external DKIE was

found.

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Table 6

Comparison of thermal stabilities of DNAZ and dihydrazido tetrazine salts

Acid pKa X� DNAZþ Hz2Tzþ

k (s�1) at 140 8C DSC exo (8C) k (s�1) at 140 8C DSC exo (8C)

HClO4 >�7 ClO4� aq. (1%) dec. 4.80E�05 191, 288 ClO4

�

HCl �7 Cl� 1.30E�03 191

HN(NO2)2 �5.62 N(NO2)2� 2.00E�03 151 1.80E�03 152 N(NO2)2

�

HNO3 �1.34 NO3� 1.30E�03 152 2.80E�03 161 NO3

�

NTO 3.67 NTO� 5.80E�03 180 2.00E�06 222 Bim-

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